Linear motors having a flat secondary and a flat, mobile primary(s) may be employed as drive means for elevators. In one linear motor embodiment, a rail fixedly mounted in the hoistway acts as the secondary and guide for the primary(s) of the linear motor. The primary(s) attaches to and drives either the elevator car or the counterweight. This embodiment advantageously fits within the hoistway, thereby eliminating the need for a separate machine room.
Each primary typically comprises a backing plate, a plurality of laminated plates and a plurality of windings. The laminated plates, which include a plurality of slots, attach to the backing plate to form the body of the primary. The slots within the laminated plates cumulatively form channels that run across the width of the primary. A plurality of windings or coils rest in the channels. Each coil is a complete path having two straight lengths connected together on each side of the primary by a coil end. One straight length of the coil rests in a first channel and the other straight length rests in a second channel separated from the first channel by a specific distance generally equal to a pole pitch.
The secondary comprises a ferromagnetic material commonly having a rectangular shape. The width and length of the secondary define the faces of the secondary. The width of the secondary is in register with the width of the primary(s). A layer of highly conductive material is fixed to each secondary face. When current passes through a coil, a magnetic field is created around the coil, perpendicular to the direction of the current. The magnetic field accesses the ferromagnetic material of both the primary and the secondary, thereby creating an attractive force between the primary and secondary.
The current in the primary coils also produces oppositely directed induced currents within the highly conductive layer fixed to the secondary. The induced currents interact with the magnetic field created by the current passing through the primary coils to create a thrust force on the primary. The strength of the induced current is directly related to the strength of the thrust force on the primary. Therefore, it is advantageous to minimize any resistance to the induced current.
It is known in the art that induced currents in the highly conductive layer run parallel to the current in the primary coils. Parallel current patterns running lengthwise in a multi-section secondary present two problems. First, electrical connections between the sections of the secondary are required. A parallel pattern running lengthwise along a secondary becomes discontinuous as it passes a seam, thereby negatively effecting the integrity of the motor, unless electrical connectors connect the sections. Second, parallel induced currents mirror the relatively long current path in the primary coils. As in most materials, the resistivity of the highly conductive layer is a linear function. The overall resistivity of the induced current path, therefore, increases with the length of the path.
In sum, what is needed is a way to minimize the amount of resistance the induced currents encounter.